Microstimulators are small, surgically implantable electrical microdevices that pass a small electrical signal to living tissue in order to elicit a response from a nerve or muscle. Microsensors are similar electrical microdevices except that they detect electrical and other signals that are generated by living tissue. The term microstimulator is intended to apply equally to both microstimulators and microsensors. The use of microstimulators or microsensors which are implanted in living tissue to stimulate a muscle function by either stimulating a nerve or the muscle itself are well known. The microstimulators receive power and control signals by inductive coupling of magnetic fields generated by an extracorporeal antenna rather than requiring any electrical leads. See for example, U.S. Pat. Nos. 5,193,539; 5,193,540; 5,324,316; 5,405,367; 6,175,764; 6,181,965; 6,185,452; 6,185,455; 6,208,894; 6,214,032; and 6,315,721, each of which is incorporated in its entirety by reference herein. These microstimulators are particularly advantageous because they can be manufactured inexpensively and can be implanted by minimally invasive injection. Additionally, each implanted microstimulator can be commanded, at will, to produce a well-localized electrical current pulse of a prescribed magnitude, duration and/or repetition rate sufficient to cause a smoothly graded contraction of the muscle in which the microstimulator is implanted.
Microdevices, as exemplified by the BION® of Advanced Bionics Corporation, are typically elongated devices with metallic electrodes at each end that deliver electrical current to the immediately surrounding living tissues. The electronic circuitry and inductive coils that control the electrical current applied to the electrodes are protected from the body fluids by a hermetically sealed capsule. This capsule is typically made of a rigid dielectric material, such as glass or ceramic, which transmits magnetic fields but is impermeable to water.
Often, while placing the miniature microstimulator in living tissue, the orientation of the microstimulator changes slightly such that the microstimulator is not in fact in electrical contact with the nerve, requiring reorientation of the microstimulator. The microstimulator may move at any point in the surgical implantation procedure. If the microstimulator has moved, it may be at a significant distance from the nerve that is to be stimulated. Consequently, more energy is needed from the microstimulator to stimulate the nerve, unless the microstimulator is repositioned closer to the nerve. While such microstimulators may be injected, the actual placement requires first locating the desired end point at a nerve or in a muscle. The method of placement involves locating the nerve with an electric probe, placing a hollow implantation tool over the electric probe and removing the electric probe to allow the microstimulator to be passed down the length of the hollow implantation tool. The implantation tool is then removed, leaving the microstimulator implanted at or near the desired location. If there is a problem with the function or location of the microstimulator, then additional surgery must be performed to remove or relocate the microstimulator, imposing risk, discomfort and potential tissue damage to the patient.
Therefore, it is desired to have a method of implantation that allows removal of the microstimulator post-implantation to allow it to be surgically re-implanted with minimal tissue disruption during its removal.